For many years conventional telephone handsets have been powered from a central office (CO) or a wiring closet of a private branch exchange (PBX). Power from the CO is supplied over the same twisted pair as is used for voice communication between the CO and the handset. In other words, the power distribution and voice communication medium comprises a single twisted pair within the PSTN (public switched telephone network). Because conventional telephone handsets are powered independent of the electrical power grid, they are able to continue functioning even when power on the electrical power grid is disrupted, for example as may happen due to a power outage.
It would be desirable to extend the advantage of electrical power independency to modern telephonic devices such as, for example, IP (Internet Protocol) telephones or IEEE 802.11 access points. (IEEE is an acronym for Institute of Electrical and Electronics Engineers; 802.11 is a wireless local area network standard set by the IEEE.) However, due to the relatively larger power demands of such devices, regulatory limits on the power that may be transmitted over PSTN twisted pair cabling, and the high resistance of PSTN twisted pairs (which can be up to 1300 Ω or 250 Ω/km for a given pair), solutions to extend the advantage have not been feasible.
Power over other types of transmission media has been addressed. For example, “Power over LAN (Local Area Network)”, known in the industry as the IEEE 802.3af standard, specifies distributing power over Ethernet cabling to remote devices on a LAN. So, in a LAN environment IP telephones may be powered independently of the status of the electrical power grid. Among other specifications of the 802.3af standard, power may be delivered over two adjacent pairs of a Category 5 cable to a remote device by applying a common mode voltage on each pair and using the differential between the pairs to deliver the power. This use of two adjacent pairs permits additional power to be distributed to a remote device that may require more power than can be delivered over a single cable pair.
Because of the problems of transmitting power over the PSTN (public switched telephone network), and to the PSTN, e.g., private branch exchange (PBXs), however, DSL (digital subscriber line) and DSL-like devices are not within the scope of the 802.3af standard. A principle reason for this is that traditional DSL operates over the PSTN and at long distances and, therefore, does not provide an efficient or even feasible means for transmitting power to a remote device. DSL devices typically require more power than a PSTN conducting pair is capable of carrying or is permitted to carry due to regulatory concerns. Indeed, in most instances the PSTN pair used for traditional DSL is not even able to carry the current that would be necessary to power the DSL interface of a DSL device, not to mention the current that would be required to power the remaining technology making up the DSL device.
At the expense of reach for increased data rates, a newer technology referred to as Long-Reach Ethernet or VDSL (Very High Bit Rate DSL) technology uses shorter cabling distances than traditional DSL, e.g., less than about 1.5 km compared to less than about 3.7 km for traditional ADSL (Asynchronous Digital Subscriber Line) technology. Because VDSL uses shorter cabling distances, distributing power over a single twisted pair to a remote DSL device, IP telephone or other remote device, becomes a near possibility. Unfortunately, however, the distances are still too great for most applications, particularly at the maximum reach of VDSL. To increase the power reach, one solution might be to use two adjacent twisted pairs to distribute power, e.g., in a manner analogous to that which may be done to transmit power over Ethernet cabling according to the IEEE 802.3af Power over LAN standard. However this approach is not practicable since adjacent pairs of a DSL system from the wiring closet or CO, i.e., from the line termination (LT) of the system, are arbitrarily routed so that there is no guarantee that they maintain their adjacency at the network termination (NT) of the system.